There has been much attention directed to planar or integrated transformers using PCB boards or semiconductors over the last ten years. Planar transformers are manufactured using a combination of embedded or attached ferrite materials and PCB techniques to improve the winding coupling. In the case of semiconductors, attempts are made to integrate the entire inductor or transformer structure into a CMOS device. Both of these methods have severe limitations, which restrict their use to low speed or narrow band applications. In the case of planar transformer designs prior art approaches fail to adequately address a method of arranging the windings to control leakage inductance and winding capacitance and its associated fabrication. As result, prior art planar transformers have poor return loss and insertion loss over a wide frequency range and are not functional and usable in many communication standards today. Transformers based on this prior art consistently fail to meet the technical requirements for data communications and are restricted to relatively low speed applications such as switching power supply systems. Integrated transformers are limited at the lower end of the band by the self inductance that primarily comes from a magnetic ferrite core with high permeability. Integration of magnetic materials onto Si tends to be difficult. Hence, Silicon transformers typically rely only on natural electromagnetic coupling and therefore typically provide narrow band performance at RF. Furthermore, integrated transformers suffer parasitic eddy currents generated by the magnetic fields in the silicon and have limited high frequency performance. As a result, integrated transformers typically have narrow band-pass characteristics and are only good for narrow frequency balun applications commonly found in wireless applications such as cellular phones. Telecom transformers require a bandpass response with a bandwidth from DC to as high as several GHz along with center taps used supplying DC power or for terminating common mode currents to reduce electromagnetic interferences. These center taps make it very difficult to achieve wide bandwidth performance.
Unlike low speed applications typically found in switching power supplies or narrow band applications typically found in wireless applications, networking and telecommunication applications typically use all of the available bandwidth in order to efficiently transfer data. Networking and telecommunications markets require linear wideband performance from near DC to multi-Gigahertz with very low loss and minimal reflected energy. Furthermore, the permeability of magnetic cores decreases as frequency increases into Gigahertz where new multi-gigabit communications applications demand the bandwidth. To compensate for the loss of magnetic coupling, the number of winding turns is increased. The increase in the number of turns induces significant leakage inductance and winding capacitance that degrade the transfer of energy and reflect significant energy. Designing multi-Gigahertz transformers to meet these stringent specifications requires several diverse techniques to be incorporated into the arrangement of the windings and associated fabrication of the planar design.
In addition, these devices must be manufactured in a manner that prevents them from breaking down in the presence of high voltage (>1500V) as electrical isolation is a critical reason why these devices are placed in series with the communications channel.
Accordingly, there is a need in the art to develop a transformer that can provide low reflected energy and electrical loss from DC to GHz for high-voltage DC isolation and low frequency common-mode rejection requirements of gigabit communications. It would be considered an advance of the art to arrange the windings that specifically control winding capacitance and leakage inductance to, lower reflected energy and extend the bandwidth from DC to GHz. Furthermore, these winding techniques and associated fabrication allow for GHz coupling where the permeability of a ferrite core drops drastically.